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    <script>
        // undefined就会触发函数参数的默认值。
        // [1, undefined, 3].map((x = 'yes') => x);
        // [ 1, 'yes', 3 ]
        let hh=[1, undefined, 3].map(function(x = 'yes'){
            console.log(x);
            return x
        })
        console.log(hh);
    </script>
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